Laws of Motion and Law of Conservation of Mass

1. 12th
Grade
Motion and
Conservation
Laws
LESSON 2:
ABM-Wohler

2. Learning Objectives
1. Explain each of Newton’s three laws of motion
2. compare and contrast the Aristotelian and Galilean
conceptions of motion
3. Use algebra, Newton’s 2nd Law of Motion, and Newton’s
Law of Universal Gravitation to show that, in the absence of
air resistance, objects close to the surface of the Earth fall
with identical accelerations independent of their mass.
4. Explain mass, momentum, and energy conservation.
5. Use the law of conservation of momentum to solve one-
dimensional collision problems

3. Aristotle on Motion
Natural motion on Earth was thought to be either
straight up or straight down.
• Objects seek their natural resting places:
boulders on the ground and smoke high in the
air like the clouds.
• Heavy things fall and very light things rise.
• Circular motion was natural for the heavens.
• These motions were considered natural–not
caused by forces.

4. Aristotle on Motion cont.,
Violent motion, on the other hand, was imposed
motion.
• It was the result of forces that pushed or
pulled.
• The important thing about defining violent
motion was that it had an external cause.
• Violent motion was imparted to objects.
• Objects in their natural resting places could not
move by themselves.

5. Galileo on Motion
One of Galileo’s great contributions to physics was
demolishing the notion that a force is necessary to
keep an object moving.
A resistive force acts between materials that touch
and slide past each other.
• Resistance is caused by the irregularities in the surfaces of
objects that are touching.
• Even very smooth surfaces have microscopic irregularities that
obstruct motion.
• If resistance were absent, a moving object would need no force
whatever to remain in motion.

6. Newton’s
Laws of
Motion
I. Law of Inertia
II. F=ma
III. Action-Reaction

7. While most people know what
Newton's laws say, many people
people do not know what they
mean (or simply do not believe
what they mean).

8. Newton’s Laws of Motion
 1st Law – An object at rest will stay at
rest, and an object in motion will stay in
motion at constant velocity, unless acted
upon by an unbalanced force.
 2nd Law – Force equals mass times
acceleration.
 3rd Law – For every action there is an
equal and opposite reaction.

9. 1st Law of Motion
(Law of Inertia)
An object at rest will stay at
rest, and an object in motion
will stay in motion at constant
velocity, unless acted upon by an
unbalanced force.

10. 1st Law
 Inertia is the
tendency of an
object to resist
changes in its
velocity: whether
in motion or
motionless.
These pumpkins will not move unless
acted on by an unbalanced force.

11. 1st Law
 Once airborne, unless acted on by an
unbalanced force (gravity and air – fluid
friction), it would never stop!

12. 1st Law
 Unless acted
upon by an
unbalanced
force, this golf
ball would sit on
the tee forever.

13. Why then, do we observe every
day objects in motion slowing
down and becoming motionless
seemingly without an outside
force?
It’s a force we sometimes cannot see –
friction.

14. Objects on earth, unlike the
frictionless space the moon
travels through, are under the
influence of friction.

15.  There are four main types of friction:
 Sliding friction: ice skating
 Rolling friction: bowling
 Fluid friction (air or liquid): air or water resistance
 Static friction: initial friction when moving an object
What is this unbalanced force that acts on an object in
motion?

16. Slide a book across a
table and watch it
slide to a rest
position. The book
comes to a rest
because of the
presence of a force -
that force being the
force of friction -
which brings the book
to a rest position.

17.  In the absence of a force of friction,
the book would continue in motion
with the same speed and direction -
forever! (Or at least to the end of
the table top.)

18. Newtons’s 1st Law and You
Don’t let this be you. Wear seat belts.
Because of inertia, objects (including you)
resist changes in their motion. When the car
going 80 km/hour is stopped by the brick wall,
your body keeps moving at 80 m/hour.

19. 2nd Law

20. 2nd Law
The net force of an object is
equal to the product of its
mass and acceleration, or
F=ma.

21. 2nd Law
 When mass is in kilograms and
acceleration is in m/s^2, the unit of force
is in newton (N).
 One newton is equal to the force required
to accelerate one kilogram of mass at one
meter/second/second.

22. 2nd Law (F = m x a)
 How much force is needed to accelerate a 1400
kilogram car 2 meters per second/per second?
 Write the formula
 F = m x a
 Fill in given numbers and units
 F = 1400 kg x 2 meters per second/second
 Solve for the unknown
 2800 kg-meters/second/second or 2800 N

23. If mass remains constant, doubling the acceleration,
doubles the force. If force remains constant, doubling the
mass, halves the acceleration.

24. Newton’s 2nd Law proves that different masses accelerate
to the earth at the same rate, but with different forces.
• We know that objects
with different masses
accelerate to the
ground at the same
rate.
• However, because of
the 2nd Law we know
that they don’t hit the
ground with the same
force.
F = ma
98 N = 10 kg x
9.8 m/s/s
F = ma
9.8 N = 1 kg x
9.8 m/s/s

25. Check Your Understanding
 1. What acceleration will result when a 12 N net
force applied to a 3 kg object? A 6 kg object?
 2. A net force of 16 N causes a mass to
accelerate at a rate of 5 m/s2. Determine the
mass.
 3. How much force is needed to accelerate a 66
kg skier 1 m/sec/sec?
 4. What is the force on a 1000 kg elevator that is
falling freely at 9.8 m/sec/sec?

26. Check Your Understanding
 1. What acceleration will result when a 12 N net force
applied to a 3 kg object?
12 N = 3 kg x 4 m/s/s
 2. A net force of 16 N causes a mass to accelerate at a
rate of 5 m/s2. Determine the mass.
16 N = 3.2 kg x 5 m/s/s
 3. How much force is needed to accelerate a 66 kg
skier 1 m/sec/sec?
66 kg-m/sec/sec or 66 N
 4. What is the force on a 1000 kg elevator that is falling
freely at 9.8 m/sec/sec?
9800 kg-m/sec/sec or 9800 N

28. 3rd Law
 For every action, there is
an equal and opposite
reaction.

29. 3rd Law
According to Newton,
whenever objects A and
B interact with each
other, they exert forces
upon each other. When
you sit in your chair,
your body exerts a
downward force on the
chair and the chair
exerts an upward force
on your body.

30. 3rd Law
There are two
forces resulting
from this
interaction - a force
on the chair and a
force on your body.
These two forces
are called action
and reaction forces.

31. Newton’s 3rd Law in Nature
 Consider the propulsion
of a fish through the
water. A fish uses its fins
to push water backwards.
In turn, the water reacts
by pushing the fish
forwards, propelling the
fish through the water.
 The size of the force on
the water equals the size
of the force on the fish;
the direction of the force
on the water (backwards)
is opposite the direction
of the force on the fish
(forwards).

32. 3rd Law
Flying gracefully
through the air, birds
depend on Newton’s
third law of motion. As
the birds push down on
the air with their
wings, the air pushes
their wings up and
gives them lift.

33.  Consider the flying motion of birds. A bird flies
by use of its wings. The wings of a bird push air
downwards. In turn, the air reacts by pushing
the bird upwards.
 The size of the force on the air equals the size
of the force on the bird; the direction of the
force on the air (downwards) is opposite the
direction of the force on the bird (upwards).
 Action-reaction force pairs make it possible for
birds to fly.

34. Other examples of Newton’s
Third Law
 The baseball
forces the bat
to the left (an
action); the bat
forces the ball
to the right (the
reaction).

35. 3rd Law
The reaction of a rocket is an
application of the third law of
motion. Various fuels are
burned in the engine,
producing hot gases.
The hot gases push against
the inside tube of the rocket
and escape out the bottom of
the tube. As the gases move
downward, the rocket moves
in the opposite direction.

36. Newton’s Law of
Universal Gravitation
 The apple was attracted
to the Earth
 All objects in the
Universe were attracted
to each other in the same
way the apple was
attracted to the Earth

37. Newton’s Law of
Universal Gravitation
 Every particle in the Universe attracts every
other particle with a force that is directly
proportional to the product of the masses and
inversely proportional to the square of the
distance between them.
2
2
1
r
m
m
G
F 

38. Universal Gravitation
 G is the constant of universal gravitation
 G = 6.673 x 10-11 N m² /kg²
 This is an example of an inverse square law
 Determined experimentally
 Henry Cavendish in 1798
2
2
1
r
m
m
G
F 

39. Conservation
Laws
1. Energy
2. Mass
3. Linear Momentum

40. KE and PE
 In many situations, there is a conversion
between potential and kinetic energy.
 The total amount of potential and kinetic
energy in a system is called the
mechanical energy
 Mechanical energy = PE + KE

41. Mechanical Energy
 The mechanical energy does not change
because the loss in potential energy is
simply transferred into kinetic energy.
 The energy in the system remains
constant!!

42. The Law of Conservation of
Energy
 The Law of Conservation of Energy states
that energy cannot be created or
destroyed.
 The big picture… the total energy in the
universe remains constant.

43.  But how? If I stop pumping while I’m
swinging, I stop!! So, where’s the
energy?

44. Conservation of Energy
 You need to remember friction…
 As you slow down on the swing, the hooks
and the chain rub against each other and
air pushes against the rider.

45.  Friction causes some of the mechanical
energy of the swing to change to
thermal energy and the temperature of
the hooks and chain heat up a little.
 The energy is still there, just in a
different form!!

46. Conservation of Energy
 Energy is transformed… not destroyed!!

47. Understanding the Law
of Conservation of Mass

48. Who Discovered this Law?
 1789, France
 Antoine Lavoisier
 Nobleman
 Statesman
 Scientist
 Used one of the first analytical mass balances
to prove this law.
 Executed on the guillotine during the French
Revolution.
 He is known as the “Father of Chemistry”
because he made it a quantitative science.

49. What does the Law of
Conservation of Mass State?
 During any chemical reaction, matter is
neither created nor destroyed. Mass is
conserved from reactants to products.
 Therefore,
MASS REACTANTS = MASS PRODUCTS

50. Linear Momentum
 This is a new fundamental quantity, like force, energy.
It is a vector quantity (points in same direction as
velocity).
 The linear momentum p of an object of mass m
moving with a velocity v is defined to be the product
of the mass and velocity:
 The terms momentum and linear momentum will be
used interchangeably in the text
 Momentum depend on an object’s mass and velocity
v
m
p




51. 3 Types of Collisions
 Elastic
 Inelastic
 Perfectly Inelastic

52. Elastic or Inelastic?
An elastic collision loses
no energy. The deform-
ation on collision is fully
restored.
In an inelastic collision,
energy is lost and the
deformation may be
permanent. (Click it.)

53. Elastic
 A collision in which two
objects move separately with
different velocities, but not
permanent deformation

54. Inelastic
 A collision in which two objects
deforms so that the objects move
in the same direction but with
different final velocities after
colliding.

55. Perfectly Inelastic
 A collision in which two objects
stick together and move with
the same velocity after
colliding.

56. For each of the
following examples,
identify the type of
collision…

57. Perfectly Inelastic

58. Perfectly Inelastic

59. Elastic

60. Perfectly Inelastic

61. Inelastic

62. Perfectly Inelastic

63. Conservation of Momentum
 In an isolated and closed system,
the total momentum of the
system remains constant in
time.
 Isolated system: no external
forces
 Closed system: no mass enters or
leaves
 The linear momentum of each
colliding body may change
 The total momentum P of the
system cannot change.